The invention relates to a roll press having an extended nip press. Such a roll press is shown, for instance, in Federal Republic of Germany DE-OS 33 11 996 (U.S. Pat. No. 4,555,305). It can preferably be used as the water-removal press in a paper making machine. The present invention concerns the control over the feeding of pressurized fluid for pressurizing the the press shoe, the inflation of the press jacket and removal of the lubricating fluid from inside the enclosure defined by the press jacket.
An extended nip press includes a stationary support member and a flexible, tubular, inflatable press jacket or shell which can be revolved around the support member. A hydraulically actuatable press shoe is supported on the stationary support member in opposition to a counter-roll, and the press shoe is pressed against the inside of the press jacket toward the press roll to press between them the fiber web being dewatered, and a dewatering felt web. There is a feed for liquid pressure to the press shoe for urging it outwardly toward the counter-roll. There is also a feed of compressed air into the interior space surrounded by the press jacket for inflating the press jacket. The press shoe and the counter-roll of this roll press together form a press nip which is not a line nip, but which is instead relatively long in the direction of rotation, i.e. an extended press nip.
The above German Application describes that the press jacket of the extended nip press is indirectly driven merely by a felt belt, which, together with the web of paper from which the water is to be removed, travels through the press nip of the roll press. The felt belt is preferably driven by the counter-roll of the press. However, the drive can be effected by another roll over which the felt belt travels.
Furthermore, the German Application describes measures for facilitating starting the roll press from standstill by providing the greatest possible reduction in its starting torque. For this purpose, compressed air is blown into the press jacket. This holds the press jacket a certain distance from the support member so that it cannot stick to the support member. Such sticking can be caused, inter alia, by the lubricant which is fed to the inside of the press jacket.
No further information is given in the German Application concerning the control of the feed of pressure liquid for actuating the press shoe. However, it is evident that the pressing pressure within the press nip can be varied by adjusting the pressure of the liquid which acts on the press shoe.
The above-described known roll press has proven its worth in practical use. However, further improvements are desirable in order, on the one hand, to still further reduce the drive energy necessary for starting the roll press and, on the other hand, to make the starting process more reliable, so that the operator need perform as few switching operations as possible.